1. Field of the Invention
The present invention relates to electrical safety components for the protection of equipment and appliances against overheating, and more particularly to non-resettable safety switches which, when triggered by excess heat, shut down the source of overheating by interrupting an electrical line, and which thereafter must be replaced, in order to reconnect that electrical line.
2. Description of the Prior Art
One non-resettable heat responsive safety switch of this type is disclosed in U.S. Pat. No. 3,821,685. This known device comprises a glass cylindrical envelope having lead-in wires on each end, a conical end closure being connected to one of the lead-in wires, and a matching longitudinally slotted contact shell being connected to the other lead-in wire. This contact shell remains engaged against the conical end closure as long as a packing of fusible granulate, which backs up the contact shell against a compression spring, remains solid and resists the bias of the compression spring. The latter, when allowed to expand as a result of softening and/or fusion of the granulate packing, withdraws the contact shell from the conical end losure, thereby permanently interrupting the connection between the lead-in wires.
This type of non-resettable thermal safety switch is intended to eliminate the risk encountered with conventional bimetallic switches, where the contact points can fuse together as a result of arcing, or where the temperature setting can be deliberately or accidentally distorted so as to render the device ineffective. One shortcoming of the mentioned prior art device is that, under borderline temperatures, the contact between the cooperating conical end closure and the shell, relying on the radial resiliency of the latter, becomes indefinite, instead of remaining intact or being permanently interrupted.
Additional shortcomings of the prior art device relate to its complexity of design, requiring comparatively expensive component parts and stringent quality controls in manufacture and assembly.